As environmental awareness grows in recent years, demands are increasing for solventless paints and plating-alternatives in the fields of building materials, automobile parts, cellular phones, and electric products, and there is a wider use of formable decorative sheets as a method to decorate formed products. In such circumstances, some new types of formable polyester film have been proposed. For instance, a highly formable polyester film with a specific melting point and breaking elongation has been proposed (see Japanese Unexamined Patent Publication (Kokai) No. 2001-72841 for instance). Formable films produced from a 1:1 mixture of polyethylene terephthalate and polybutylene terephthalate have also been disclosed (see Japanese Unexamined Patent Publication (Kokai) No. 2002-321277 for instance). These proposed films, however, requires a high deforming stress during the forming process and accordingly they are difficult to form into a complicated shape. Polyester films for forming components that have a specific melting point and are designed for specific film production conditions have also been disclosed (see Japanese Unexamined Patent Publication (Kokai) No. 2003-211606 for instance). In addition, other studies (see Japanese Unexamined Patent Publication (Kokai) No. 2006-51747 for instance) have disclosed polyester films consisting of three laminated layers, namely Layers A, B, and C, of which Layer B, or the intermediate layer, has formability so that the films have a combination of formability, decorativeness, and smoothness. Furthermore, polyester films that consist of three layers, namely Layers A, B, and C, and require a specified forming stress at a specific melting point and specific temperature have been disclosed (see International Publication WO 2006/104116 for instance). These polyester films, however, do not have a sufficient breaking elongation and cannot serve for deep-draw forming.
There is another proposal that addresses a polyester resin composition and sheet for calendaring that contains a crystal nucleating agent and has a low melting point (see Japanese Unexamined Patent Publication (Kokai) No. 2003-201391 for instance). However, the materials described in this proposal do not have a sufficiently high heat resistance and dimensional stability, and cannot serve for forming processes.
It could therefore be helpful to provide a highly heat resistant and dimensionally stable polyester film that is so high in formability as to maintain a low stress and a high ductility during forming processes.